Yes, the kava will reach it's max kavalactone content in general in about 18-24 months, this has been confirmed by many tests. It is better to let them grow to 4-6 years old, I do find that the kavalactones do not increase very much, even if you harvest a 121 year old kava but there is a lot more mass and the mass is more mature than say an 2 year old kava because the 2 year old kava still has young stalks, and that means young stump and young lateral roots. When I say young I mean only a few months old, these have been tested and are very low in kavalactones. So letting them age more will help those young parts become old and full of kavalactones.
Another interesting point is that when we tested kava that was in the wild (over 100 years old) and kava from a farm (4 yeas old), the kava in the wild was lower in kavalactones, a lot lower than the farmed kava. We also found out that sun can increase kavalactones as well as good food for the kava plant will increase kavalactone content. Please let me know if you have any other questions about growing kava for high kavalactone content. Aloha.
Chris